In a number of applications utilizing wideband amplifiers, it is often desirable to control the gain of the amplifier electronically, without affecting any other performance parameter. This type of electronic gain control is particularly useful in communication circuits, such as radio-frequency (RF) amplifiers, to improve the signal handling capability or the dynamic range of the amplifier. The electronic gain control capability allows the amplifier gain to be controlled by an automatic gain control (AGC) loop.
With the advancements made in the realm of wireless personal communications systems (PCS), a multitude of communication protocols have been established. Examples of these protocols include digital European cordless telephone (DECT), public access communication system (PACS) protocol, personal handy phone (PHP), and code division multiple access (CDMA). These protocols in turn use a variety of modulation schemes such as quadrature phase shift keyed (QPSK) modulation for CDMA, gaussian frequency shift keyed (GFSK) modulation for DECT, and pi/4 quadrature phase shift keyed (pi/4 QPSK) modulation for PACS and PHP. In order to bring PCS products to market quickly and efficiently there has been an increasing interest in combining as many radio functions as possible into common integrated circuits. This allows the overall cost of theses products to decrease while creating a more synergistic approach to the communications market. The result has been an increased demand for common chip sets that satisfy the requirements of various communication protocols. However, issues arise when trying to meet multiple specification requirements within a single integrated circuit (IC). Such an issue exists between the dynamic range requirements of amplifier drivers known as voltage controlled attenuator (VCA) circuits and the slope of these VCA circuits.
Two important parameters of a VCA circuit are its automatic gain control (AGC) range and its slope. The CDMA protocol requires at least 60 dB of dynamic range while the PACS protocol requires an attenuation slope of 40 dB/volt. These two protocol requirements contradict each other. The problem is further complicated when a digital to analog converter (DAC), which has a predetermined resolution is used to control the attenuation of the VCA.
Accordingly, there is a need for a VCA circuit that satisfies the protocol requirements for both dynamic range and slope of various protocols within a single integrated circuit.